From the past, various speed control devices for maintaining vehicle speed at fixed levels have been known. According to such a speed control device which is sometimes called as a cruise control device, the driver is not required to keep stepping on the accelerator pedal to keep the automobile cruising at a constant speed and he is free from the need for adjusting the depression of the accelerator pedal in order to maintain a constant speed irrespective of the inclination and other conditions of the road.
Vacuum actuators which derive power from negative pressure of the engine intake system are commonly used as actuators for vehicle speed control. A conventional typical vacuum actuator comprises a diaphragm which defines a negative pressure chamber in cooperation with the casing of the actuator and a plurality of solenoid valves which selectively communicate the negative pressure chamber with the intake system of the engine or the atmosphere as required, and the resulting displacement of the diaphragm is transmitted to the accelerator pedal by way of a control cable. The solenoid valves are controlled by a control device incorporating a micro processor, and the output of a speed sensor is supplied to the control device. Thus, using the vehicle speed as a controlled variable and the accelerator pedal depression as a manipulated variable, the control device controls the accelerator pedal depression by way of the solenoid valves and maintains the vehicle speed at a constant level by a feedback control.
Specifically, negative pressure from the engine intake system is supplied to the negative pressure chamber by way of a negative pressure valve when the accelerator pedal depression is required to be increased, and the atmospheric pressure is introduced into the negative pressure chamber by way of a vent valve when the accelerator pedal depression is required to be reduced. Additionally, when the accelerator pedal is required to be quickly released, for instance when the vehicle brake is activate, a safety valve is activated and quickly communicates the negative pressure chamber with the atmosphere. Thus, in order to assure a high level of reliability, the vent valve and the safety valve are used in parallel in a redundant manner.
While the negative pressure source only draws air from the actuator, the atmospheric air supplied to the actuator through the vent valve and the safety valve must be actually introduced into the valves and eventually to the engine intake system. Therefore, in order to assure proper functioning of the solenoid valves, the atmospheric air must be filtered with an air filter unit which is typically internally equipped in the actuator. Since the air is more or less continually introduced into the actuator, the air filter unit must be capable of functioning properly for a long time period without replacing filter elements. Clogging of the air filter elements will cause a failure of the actuator.
Japanese Patent Laid-Open Publication No. 62-96144 (based on U.S. patent application Ser. No. 783,039 filed on Sept. 30, 1985 now abandoned) discloses a vacuum actuator of this type. This actuator is provided with a vent valve and a safety valve, but an air filter device having a single filter element which is common to the vent valve and the safety valve selectively communicates a vacuum chamber defined by a diaphragm with the atmosphere. However, according to this prior art, the air passage within the filter element appears to be restricted and local clogging of the filter element appears to be inevitable over an extended service period.